Intramolecular methylacridan-methylacridinium complexes with a phenanthrene-4,5-diyl or related skeleton: geometry-property relationships in isolable C-H bridged carbocations.

Bridging the gap: Snapshots of 1,6-H-shift precursors indicate that a narrower C-HC(+) separation (D in the ORTEP diagram) in the title complexes induces faster degenerate rearrangement of 1(+). A contact distance of less than 2.7 A is necessary to realize the organic three-center two-electron bond of [C-H-C](+), as indicated by extrapolation of the X-ray data.The isolation and low-temperature X-ray analyses of a series of intramolecular methylacridan-methylacridinium complexes have been achieved. The two chromophores are in close proximity due to an arylene spacer, such as a phenanthrene-4,5-diyl or biphenyl-2,2'-diyl unit. These bridged carbocations prefer the C-H localized structure both in solution and in the solid state. The bridging hydrogen atom undergoes a facile intramolecular hydride shift from one carbon atom to another in solution, and the energy barrier is linearly correlated with the intramolecular CC(+) distance in the solid-state geometry, as determined by single-crystal X-ray analyses. By extrapolation from the data, the delocalized three-center bond of [C-H-C](+) would be formed when the CC(+) distance is less than 2.7 A.